Electromagnetic wave energy absorbing material



A. R. SALTZMAN E I- 2,923,689

GNETIC WAVE ENERGY ABSORBING MATERIAL Feb. 2, 1960 ELECTROMA OriginalFiled Aug. 31

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INVENTORS ALVIN R. SALTZMAN JACK M. ROSEN BY Feb. 2, 1960 A. R. SALTZMANETA!- 2,923,689

ELECTROMAGNETIC WAVE ENERGY ABSORBING MATERIAL Original Filed Aug. 31,1953 2 Sheets-Sheet 2 DIELECTRIC CONSTANT 7'/ME (HOURS) 4' Q nfl I x. af a 400 i q AT 500 k TIME (HOURS) Fig. 4 k INVENTORS ALVIN R. SALTZMANJACK M. ROSEN A TTORNEYS United States Paemo" ELECTROMAGNETIC WAVEENERGY ABSORBING MATERIAL Alvin R. Saltzman, Willow Grove, and J ack M.Rosen,

Levittown, Pa.

Original application August 31, 1953, Serial No. 377,745,

now Patent No. 2,837,720, dated June 3, 1958. Divided and thisapplication December 6, 1956, Serial No. 626,785 r a 4 Claims. (Cl.252-507) (Granted under Title 35, US. Code (1952), sec. 266) Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the'payment of any royalties thereon or therefor.

This application is a continuation-in-part of. applicants co-pendingapplication Serial No. 466,461, entitled Electro-magnetic Wave EnergyAbsorbing Material, filed in the United States Patent Oflice on No-'vember 2, 1954, now abandoned, a divisional application ofapplicants copending application Serial No. 377,745, entitled Attenuation Device andMaterial, Therefor, filed in the United States Patent Olfice on August31,

1953, and issued June 3, 1958 as U.S.. Patent No.

The wide use of dummy loads and similar attenuating devices'inconnection with high frequency wave transmission emphasizes theimportance of an efficient energy absorbing material for use in suchdummy loads. A common difiiculty attendant upon the use of. variousknown types of dummy loads and relateddevices' is predicated upon thefact that wave energy reflection is produced by the presence of theenergy absorbing ma- 2,923,689 Patented Feb. 2, 1960 A furtherdisadvantage of known types of dummy loads is the factthat the energyabsorbing material used in them characteristically does not provide alinear dissipationofeneigy throughout the load, thus resulting in anuneven distribution of temperature with the consequent formation of hotspots and accompanying non-uniform local expansion of material tendingto cause local separation of the energy absorbing material from theadjacent wall of the dummy load and arcing'in the separated areasbetween the respective surfaces.

It, therefore, is an object of this invention to provide anelectro-magnetic wave energy absorbing material adapted to absorbsubstantial amounts of wave-signal energy vin a relatively small spaceandvhaving a wave impedance substantially purely resistive.

It isanother object of the invention to provide'an electromagnetic waveenergy absorbing material, adapted to be interposed in' anon-dissipative wave propagation path having a predetermined waveimpedance, which is capable of absorbing substantial amounts of waveenergy, yet has a substantially purely resistive wave impeda'nce equalto that of the non-dissipative wave propagation path.

I, It 'isstill another object of this invention to provide an:energyyabsorbing material for dummy loads which producesa lineardissipation. of energy throughout the field'energies separately'aresubject to undesirable space requirements and'excessive expense.

Attempts .to minimize reflection in wave guides, for example, by shapingthe resistive materials to a form selected to minimize reflection aresatisfactory. to some degree. However, regardless of the shape of theresistive material, it is diflicult to avoid reflection in a limitedspace such as that within a wave guide. This is because substantialenergy dissipation in a limited space requires such large values ofenergy attenuation per .wave length of a translated wave thatdissipation of either the electric field energy or the magnetic fieldenergy by itself causes a prohibitive amount of reactive component inthe wave impedance presented in the translated wave. Accordingly, in theinterest of favorable space requirements, the best approach to thisproblem appears to be an arrangement which provides for dissipation ofthe two kinds of reactive components caused by the electric field energyand magnetic field energy dissipations. One such arrangement is a waveguide having incorporated therein an electromagnetic wave energyabsorbing material having a composition which willeffect dissipation ofthe reactive components by cancellation of the respective reactivecomponents; describedtabove.

load, insuring uniform expansion of this material in orderto'avoid thetendencycharacteristic of known materials tovex'pand irregularly due tothe formation of local hotspots and the attendant increased localexpansion, resulting in, separation of the material from the dummy loadwall and the undesirable arcing which occurs as a result of'thi'sseparation.

Afinal objectofthis invention is the provision of an energy, absorbingmaterial for dummy loads characterized, by its high resistance to theadverse efiects of exposure to .extremely hi h temperatures :forextended periods. i i

vThe exact nature of this. invention as well as other objects andadvantages thereof will be readily apparent from consideration of thefollowing specification and the annexed drawings in which:

.Fig. 1 is a graphical representation of the efiect of baking time onthe dielectric breakdown of silicone films,

Fig. 2 is a graphical representation of the effect of baking time on thedissipation factor of silicone films,

and

'Figs. 3 and 4 are graphical representations of the effect of bakingtime on the dielectric constant and the capacitance, 'respectively, ofsilicone films. 1

In view of the intended use of the material of the instant invention, itis essential that it demonstrate favorable properties under electricalstress, including dielectric breakdown, dissipation factor, anddielectric constant; as well as desirable mechanical properties such asflexibility and adhesion. Moreover, taking into account the relativelyhigh temperature levels attained within a dummy load incorporating thismaterial, due to the attenuation of substantial quantities of waveenergy within a closely confined space, it is also essential that thematerial, and particularly the binder selected therefor, be capable ofwithstanding high temperatures over extended periods withoutdeterioration of the requisite electrical and mechanical propertiesenumerated above.

The electromagnetic wave energy absorbing material of this inventioncomprises an isotropic medium having a high dielectric constant siliconeresin binder composed of a polyorganosilioxane such aspolymethylsilioxane, for example, into which is incorporated a quantityof particulate conductive material such as iron, aluminum or the like, aquantity of particulate resistive material tained in a dummy load, it isalso essential that the binder selected for use in the material of theinstant in vention be capable of withstanding high temperatures overextended periods without deterioration of the critical propertiesenumerated above. In addition, it is desirable that the binder useddemonstrate some fiexibility as well as good adhesion under continuinghigh temperature conditions. a I

Since all of these requirements are. satisfied by certain siliconeresins, a material of this type has been specified as the binder in thematerial of the instant in-. vention. More particularly, the binder maybe any polyorgano'silioxane, such as polymethylsilioxane whichdemonstrates the requisite properties specified above.

A paper by A. H. Saltzman entitled Eifects of Heat on the Electrical andMechanical Properties of Silicone Films, and dated July 15, 1948,reporting on an investigation dealing with a technique for evaluatingvarious properties of silicone films, discloses the results of a seriesof different tests conducted at different temperature levels and overvarious time intervalsupon speci? mens or a typicalsilicone material ofthe type specified above, Dow corning sili'cone Resin No 993. Varioussignificant electrical properties of thisparticular silicone are clearlyindicated by the test results illustrated graphically in Figs. 1 through'4 of this application.

The respective curves in Fig. 1 indicate the variation with time indielectric breakdown in volts/mil of a thin film of the silicone tested,at temperatures of 300, 400 and 500 degrees Fahrenheit. It will be notedfrom a comparison of the curves shown that the dielectric breakdownreaches the highest level at .400 and remains substantially unchanged bycontinued exposure to high temperature after the first 1000 hours.

The two curves in Fig. 2 indicate the variation with time of thedissipation factor in percent at temperatures.

of 400 and 500 degrees Fahrenheit. In this connection, the dissipationfactor can in this case be taken as equal to the power factor, since thepower factor, loss angle and dissipation factor are for all practicalpurposes equal when the values are small. From the showing in Fig. 2 itis apparent that the dissipation factor or powerfactor reaches arelatively stable minimum value of approximately .2 upon exposure to a400 temperature and remains substantially unchanged'during continuedexposure to this high temperature.

The respective curves in Fig. 3 indicate the variation with time of thedielectric constant of the silicone tested, at temperatures of 400 and500 degrees Fahrenheit, respectively, While the respective curves inFig. 4 indicate the variation with time of the capacitance of thesilicone tested, at temperatures of 400 and 500 degrees Fahrenheit,respectively. With respect to each of these properties, it will be notedthat they remain substantially con stant over an extended period of morethan 1200 hours at 400 While they vary substantially over'a relatively 4though at temperatures approaching 500 degrees Fahrenheit the cumulativeeffect of the various properties on the characteristics of the materialbecomes less favorable.

The paper referenced above also reported the results of tests todetermine the flexibility and adhesion of a typical silicone resinexposed to high temperature. These tests were conducted upon thespecimens, consisting of films of silicone resin adhered to copperpanels, after they had been used to obtain the test results illustratedgraphically in Figs. 1 through 4 by being exposed to the specifiedtemperatures continuously for the extended time intervals indicated. Thevisually determined results of these latter tests involvingpredetermined uniform fiexure of the various specimen panels indicatedthat the flexibility of silicone films of the type tested remains goodwhether the silicone films are heated at 300, 400, or 500 degreesFahrenheit. However, at 500 degrees and after 575 hours a of heating atthis temperature, the films showed cracks ;-s'i en- 'less thanthe'wav'elen gth of the electromagnetic wave being translated, are used.The individual par ti'cles maybe of relatively regular shape, butthe(2on ductivepa'rticles preferably have a plate-like substantially oblongor ellipsoid shape, the long dimension being very large. compared to theshort dimension, ofthe order of 5 are 01.. Iron particles of this shapemay be pun. chasedor they may be made by dampening particles of,relatively "uniform dimensions to wetness and processing, themin acolloidal mill.

. The use of oblong shaped conductive particles inthe, pfeferr'edmodification of the instant invention has a, number of advantages, asset forth in the above referred to co-pending application, Serial No.377,745, 'now US,

Patent No. 2,837,720, a backing material may be coated;

withv the material of the instant invention in tacky condition and thenbe suspended so that a major portion of the particles are aligned bysurface tension with their long.axes substantially parallel.Alternatively, if the particles are of magnetic material their alignmentmay be, accomplished.magnetically. This alignment has thefad-v vantagesthatmorev efiicient spatial arrangement is; ac-. complished with theparticles closer together and it ris sures the preferred alignment ofthe particles inan at-y tenuation element made from the backingmateriahwith, respect to electric and magnetic field vectors of theelectro:v magnetic wave being translated. For maximum efficiency in thepreferred modification using oblong shapedparticle s, the particles ofmagnetic material when incorporated in an attenuation device should bealigned with their long dimensions normal to the vector of theelectricfield of the translated wave and the resistive -particles should bealigned with their long dimensions normal to the vector of the magneticfield.

The additional dielectric material specified; above, namely, bariumtitanate, in a finely powdered form, is uniformly dispersed throughoutthe mixture. Due to its-- high dielectric properties, its presencefacilitates theat: tainment "of the desired absorbing properties of'anele-. ment incorporating the material of the instant invention. Inaddition, the inherent ferroelectricproperties of barium titanate arealso believed to enhance the :energy absorbing properties of the dummyload. Through the useof this compound, most desired ratios of dielectricconstant to magnetic permeability are readily obtained for-thematerial.The barium titanate also serves to spread the dissip'ation of heatuniformly over a greater volumeof the dissipation material, thusreducing-the possibility;

bi "hot spots "due to an accuiiinlationof power in a small volume, andthe consequent variation m the local rate of expansion of the materialdue'to these -hotspotsf which results in separation of-the material fromthe sur- 7 face to which it is attached. In fact, the uniformdissipation of energy in the material characterizing the instantinvention and the attendant uniform temperature level throughout thismaterial giving rise'to' a uniform rate of expansion therefor isenhanced as a contributing factor in assuring a good bond between thismaterial and the surface of a metallic w'all'in which it is mounted,such as that of a wave guide, by the fact that this materialcharacteristically has a coefiicient'of expansion substantially the sameas that of the metallic wall, particularly when the Wallis made of brasswhich is most commonly used in wave guide construction.

In preparing the material of' the instant invention, a mixture is madeat room temperature of appropriate quantities of the resistive andconductive particles with the bariumtitanate and the mixture is thendried. In this connection, it is to be noted that the relative amountsof dielectric material and conductivematerial are so' proportioned withrespect to their'magnetic permeabilities and dielectricconstants as togive a ratio substantially equal to a like ratio of the wavepropagationpath'for the mode of wave propagation. The proportions may bevaried to produce various ratios. A purely resistive wave impedance-ofthe energy absorbing material is obtained by proportioning the relativeamounts of resistive and conductive material so that equal amounts ofmagnetic field energy and electric field energy dissipation areobtained, as the two kinds of dissipation tend to cause 'oppositereactive components in the wave impedance.

For the purpose of mixing the material thesilicone resin binder may bethinned to the proper consistency by using weight of toluene. Then thesilicone res in binder is added to the dried mixture at room temperatureand the resulting product is milled in a ball'mill to form a suspensionof'the various particles in the. resin. When. the

energy absorbing material of the instant inventionis.

formed in thisv manner, the particles of resistive andcon ductivematerial and the particles of barium titanate are 'held-in solidsuspension andare individually-insulated by being separated by minutedistances to provide space occupied by the high dielectric siliconeresin binder.

As noted above, the proportions of the various components of thismaterial may be varied as necessary to achieve the desired performancecharacteristics for various applications. However, the percentages ofthe respective ingredients by weight in relation to the weight of thesilicone resin used are 1 to percent iron particles and 12 to 18 percentof carbon particles mixed with 10 to percent of barium titanate forcombination with the silicone resin binder. More specifically,satisfactory results have been obtained with a mixture of 1 percent ironparticles and 12 percent carbon particles mixed with 25 percent bariumtitanate and combined with the silicone resin known as Dow CorningSilicone Resin No. 993. However, in view of the substantial variationsin the characteristics of materials such as the Dow Corning SiliconeResin No. 993 as between various batches and further variations in thesecharacteristics depending upon the age of the material, the exactproportions of the varying degrees used must presently be determinedexperimentally for each quantity of such resin used taking into accountits condition at the time it is used, pending the possible establishmentof more rigid control of the characteristics of this type of resin.

Dummy loads incorporating the material described .herein, mixed in theparticular proportions indicated above. gave a voltage standing waveratio of 1.05 or better. i In addition, a dummy" load containing fourcubic inches of the material gave 80 to'lOOwatts dissipation or about 25watts per cubic inch. Moreover, the material of the instant-inventionmay be used in various attenuation applications. For example, thematerial itself may be molded and formed into various shapes or it maybe applied as a coating to a surface such as the ing a substantiallypure wave impedance so that it does -not reflect wave-signal energy.

Obviously many modifications and variations of 'the present inventionare possible in the light of the above teachings. It is, therefore, tobe understood that Within the scope of the appended claims'the inventionmay be practiced otherwise than as-sp'ecifically described.

What is claimed is: 1 1. A heat resistant electromagnetic wave energy'absorbing material capable of uni-form energy dissipation over itsentire extent for use asa dummy load in'a given wave propagation pathhavinga predetermined ratio of magnetic permeability to dielectricconstant for a given mode of wave propagation; said energy absorbingmaterial consisting essentially of a silicone resin binder in theamount,

of substantially 81 percent to substantially 65 percent of the totalweight of the-waveenergy absorbing material, a quantity of finelydivided and evenly distributed dielec- -tric material composed of bariumtitanate in the amount of 10 to 25 percent by weight of said resinmixedwith the resin binder; a quantityof highlyconductive metallicmaterialincorporated in .said dielectric material in the amount of 1 to10 percent by weightof said resin consisting of particles having atleast one dimension smallin relation to the wave length of anelectromagnetic wave to be translated thereby; the relative quantitiesof .saidconductive material and said dielectric material being soproportioned with respect to their magnetic permeabilities anddielectric constants as to provide, for the given'mode of wavepropagation along the given wave propagation path, a ratio of magneticpermeability to dielectric constant for said energy absorbing materialsubstantially equal to said predetermined ratio; and a substantialquantity of high resistance carbonaceous material incorporated in saiddielectric material in the amount of 12 to 18 percent by weight of saidresin.

2. An electromagnetic wave energy absorbing material capable of uniformenergy dissipation over its entire extent for use as a dummy load in agiven wave propagation path having a predetermined ratio of magneticpenneability to dielectric constant for a given mode of wavepropagation; said energy absorbing material consisting in the amount ofl to 10 percent by weight of said resin binder consisting of particleshaving at least one dimension small in relation to the wave length of anelectromagnetic wave to be translated thereby; the relative quantitiesof said conductive material and said dielectric material being soproportioned with respect to their mag netic permeabilities anddielectric constants as to provide.

"7 for; the :given' mode of wave propagation along the given wave;propagation path, a ratio; of: magnetic. permeability tor dielectricconstant for. said energy :absorbingmaterial :substantially equaltOgSflidtPICdIfII1ll1d nation and a substantiah quantity. of highresistance, carbonaceousmate- ;rial; in the amountgof l2to ;l8;percer1tbyl weightof said Jesinbinder, consisting-of particles having atleastonediomension small, inrelation to the wave length, of =an-electromagneticWaVenjtO; be ytrauslated thereby;- .the" relative quantitiesof resistiveandnconductiver-material ,being-so proportioned that equal amounts ofmagnetic field energy and electric, field energy dissipation areobtainedrinl-order to obtain a purely resistivewave impedanceofeaidfinergy absorbing material.

3r An electromagnetic wave energy absorbingmaterialcharacterizedbyuniformenergy absorption in all portions thereof or useasa dummy load in a given wave propavgation; path; having; a;predetermined ratio of, magnetic .permeabilitym; dielectric constant,for a given, mode :of wave propagation; said energy absorbing materialconsisting essentiallyof a quantity of dielectric material including athigh heat resistant T silicone resin binderrin latheamount-ofsubstantially 81; percent to substantially, 65 percent byweight, ;of,-said electromagnetic wave energy persed throughout saidsilicone resin binder in the amount of 10 to 25 percent by weight ofsaid resin-, binder; a quantity of highly conductive metallic material:of ,individually insulated particleswith maximum dimensions .small inrelation to the Wave length of the electromagnetic wave to be translatedthereby dispersed in-said'dielectric imaterial in theamount of 1 to 10percent by weight of said resin binder andefiective tocause dissipationofthe "magnetic energy of said translated wave;-; the relative amountsof said materials being so proportionedasto .give a ratio of effectivemagnetic permeability to; elfective dielectric constant for said energyabsorbing material substantially equal to said predeterminedtratio. ofmagnetic permeability to dielectric constant forthe given mo.de:of wavepropagation along the given wave propagation .path; vand a'substantialquantity of highly resistive carbonaceous {material of individuallyinsulatedparticles having maxieimum dimensions small inrelation to theWave length of -saidttranslated wave dispersed in said dielectricmaterial 1' in the amount of 12 to 18 percent by weight of said resinzbinder'and'efiective to-cause dissipation of the electric :,-energy:oft said translated wave.

absorbing material, and barium titanatei uniformly dis-,

.0 :lsionssmall in r elation to the wave length of saidtrjans'lated I 26a constant for the given mode of wave propagation along tioned]to causesubstantially .equaldissipat'i on .of th e :4. electromagnetic waveenergy absorbing material;

use as, a, dummy. load in a .given ,Wave propagation. pat

.having a, predetermined ratio ,of magnetic permeabi itym- :dielectricconstant for a given mode ot wave propagation;

said venergy absorbing material consisting essentially,of a, quantityofw dielectric material including a high heat :resi stant silicone.resin binder in-theamount of substantially 73.5 percent by weight ofsaid electromagnetic wave energy absorbing material andfinely dividedbarium titanate uniformly dispersed throughout said siliconere sinbinder in the amount of substantially 25 percent by weight of said resinbinder; 'a quantity of highly conductive materialof individuallyinsulated iron particles with maximum dimensions small in relation ,tothe wave length ,of :the electromagnetic wave to be translated thereby:,dis- ,persedin vsaid dielectric. material in the amountbftsubstantially1 percent byweight .ofsaid resin binder and effective tc-causedissipation of the magneticenergy of said translated wave; ,the relativeamounts of said matevirials tbeing so proportioned as to give a ratio ofefiective m agneticpermeability to efiective dielectric constant forsaid energy absorbing material substantially equal tosaid predeterminedratio of magnetic permeability to dielectric the given wave propagationpath; and a substantial quant :it y oifghighlyresistive materialconsistingof individ pally, insulated carbon particles havingmaximumdimenwave dispersed in saiddielectric material in the amountof;.-substantially 12 percent by weightoi said resin binder iiective to.cause dissipation -.of the electric energy H H translated Wave; therelative uantities of said conductive material and saidrresistivematerialbeing proporrnagnetic field energy and the electricfield energyof the v.t ralnislated electromagnetic wave.

References Cited in the file of this patent UNITED STATES PATENTS'2;277,736 Wainer l Mar. 31, .1942 2,526,059 aZabel et a1. 'Oct. 17,19502,579,327 IJund 'Dec'.- 18,; 1951 2,610,250 aWheeler Sept. 9, 1952 t2,658,833 ,Coffeen et al Nov. 10, 1953 2,730,597 'Podolsky "Jan. 10,1956

1. A HEAT RESISTANT ELECTROMAGNETIC WAVE ENERGY ABSORBING MATERIALCAPABLE OF UNIFORM ENERGY DISSIPATION OVER ITS ENTIRE EXTENT FOR USE ASA DUMMY LOAD IN A GIVEN WAVE PROPAGATION PATH HAVING A PREDETERMINEDRATIO OF MAGNETIC PERMEABILITY TO DIELECTRIC CONSTANT FOR A GIVEN MODEOF WAVE PROPAGATION, SAID ENERGY ABSORBING MATERIAL CONSISTINGESSENTIALLY OF A SILICONE RESIN BINDER IN THE AMOUNT OF SUBSTANTIALLY 81PERCENT TO SUBSTANTIALLY 65 PERCENT OF THE TOTAL WEIGHT OF THE WAVEENERGY ABSORBING MATERIAL, A QUANTITY OF FINELY DIVIDED AND EVENLYDISTRIBUTED DIELECTRIC MATERIAL COMPOSED OF BARIUM TITANATE IN THEAMOUNT OF 10 TO 25 PERCENT BY WEIGHT OF SAID RESIN MIXED WITH THE RESINBINDER, A QUANTITY OF HIGHLY CONDUCTIVE METALLIC MATERIAL INCORPORATEDIN SAID DIELECTRIC MATERIAL IN THE AMOUNT OF 1 TO 10 PERCENT BY WEIGHTOF SAID RESIN CONSISTING OF PARTICLES HAVING AT LEAST ONE DIMENSIONSMALL IN RELATION TO THE WAVE LENGTH OF AN ELECTROMAGNETIC WAVE TO BETRANSLATED THEREBY, THE RELATIVE QUANTITIES OF SAID CONDUCTIVE MATERIALAND SAID DIELECTRIC MATERIAL BEING SO PROPORTIONED WITH RESPECT TO THEIRMAGNETIC PERMEABILITIES AND DIELECTRIC CONSTANTS AS TO PROVIDE, FOR THEGIVEN MODE OF WAVE PROPAGATION ALONG THE GIVEN WAVE PROPAGATION PATH, ARATIO OF MAGNETIC PERMEABILITY TO DIELECTRIC CONSTANT FOR SAID ENERGYABSORBING MATERIAL SUBSTANTIALLY EQUAL TO SAID PREDETERMINED RATIO, ANDA SUBSTANTIAL QUANTITY OF HIGH RESISTANCE CARBONACEOUS MATERIALINCORPORATED IN SAID DIELECTIC MATERIAL IN THE AMOUNT OF 12 TO 18PERCENT BY WEIGHT OF SAID RESIN.